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Brain and Nervous System Basics

Originally published on November 10, 2009
Most recently updated on April 10, 2013

Together, the brain and spinal cord make up the central nervous system, also referred to as the CNS. The network of nerves that connects the CNS to the arms, legs, eyes, ears, and other organs is called the peripheral nervous system (PNS). We are usually aware of our legs and arms moving and can generally control them. But other activities—such as blood circulation, breathing, digestion, and the work of hormones in our body—are carried out without our thinking much about them. These are functions of the autonomic nervous system, which is controlled largely by the brainstem.

Because the brain and spinal cord are so vital to the body’s function and survival, nature has provided some protective armor against harm from the outside. The first layer of protection is the skin of the scalp, which plays an essential role in fending off infection. Next are the bony structures of the skull and spinal column. The top of the skull, or cranium, surrounds the brain, keeping it from being crushed; its rounded design gives the cranium added strength despite its relative thinness. The spinal cord is housed inside the hollow vertebrae, or spinal bones, of the neck and back. In addition, muscle groups strengthen and pad the spine.

Beneath the bony structures covering both brain and spinal column are three layers of membranes, called meninges. The outer layer is the dura mater, a tough, translucent skin. Next is the spongy substance called arachnoid, which contains blood vessels. Closest to the brain surface is the pia mater, which contains major blood vessels and covers the brain’s wrinkles and folds.

You will hear the abbreviation CSF frequently. It stands for the term cerebrospinal fluid. CSF is a clear, watery substance that forms yet another protection for the brain and spinal cord, cushioning them from jolts and knocks much as amniotic fluid protects a fetus, nourishing the brain, and carrying away waste products. CSF is found between the pia mater and arachnoid layers of the meninges, as well as throughout the brain in cavities and tunnels called ventricles. It is constantly manufactured in a place within the ventricle called the choroid plexus. The CSF then circulates through the ventricular system to the subarachnoid space, where it is reabsorbed into the blood stream.
                       
Although CSF’s essential function is beneficial, the body can be harmed when a brain tumor blocks the flow of CSF from a ventricle or if too much CSF is produced. If that happens, fluid builds up within the brain, which has very little room to expand inside the skull, resulting in raised intracranial pressure (ICP). The resulting condition is called hydrocephalus. A child may experience one or more of the following symptoms: headaches, vomiting, clumsiness, and drowsiness or lethargy. These symptoms may be the first indication of the presence of a brain tumor.

You will also hear about the blood–brain barrier, which is a cellular layer, or layer of cells, that protects the sensitive brain from fluctuations in the chemistry of the blood flowing through the cerebral blood vessels. The blood–brain barrier is essential to the brain’s survival and health. It also prevents therapeutic drugs and antibiotics from reaching tumors and infections in the brain. Researchers continue to look for ways around this barrier when it is standing in the way of treatment.

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